This invention relates to printed circuits and, more particularly, to a method for interconnecting flexible printed circuits.
Generally, printed circuit layers are mechanically connected by fastening two circuits together in the proper relationship on a substrate by means of an adhesive. Electrical interconnections are then accomplished by various methods, such as by punching holes through both printed circuit boards and plating through these holes to connect the conductor layers of the two printed circuits. Besides plating through the holes, eyelets, tublelets, welding, brazing, etc. have been used in the holes to establish the interconnections. In still other processes, the connecting holes are filled with conductive material such as powdered metal (followed by sintering), metal paste or conducting paint. These processes are expensive and time consuming because, except in the plated-through hole process, each interconnection hole must be processed individually. Although the plated-through hole process is a mass production process which does not require that each interconnection be treated individually, it has disadvantages because the plated-through holes are subject to plating cracks when the boards are flexed, thereby producing intermittent connections.
Recent advances have led to the successful use of solder to interconnect printed circuits, thereby making use of the wave soldering step, which connects circuit components to the board. Such a process is described in U.S. Pat. No. 3,888,629 to Hastings et al., which is assigned to the assignee of the present invention. This process makes use of concentric holes of different diameter at each interconnection point in order to permit the solder to flow through the holes connecting the two circuits. With the concentric hole technique it is sometimes difficult to register all of the holes needed for a complex circuit. Also, this type of connection cannot be visually inspected.